1. Field of the Invention
The present invention pertains to spread-spectrum signal acquisition and tracking and, in particular, to direct-sequence spread-spectrum digital signal acquisition having predictive tracking with clock drift adjustment.
2. Statement of the Problem
A recognized problem with spread-spectrum communication is the acquisition of the spread signal by a receiver and, after acquisition, the continued tracking of the spread signal for proper data reception. The term "spread-spectrum" is defined as any of a group of modulation formats in which an RF bandwidth much greater than necessary is used to transmit an information signal so that a signal-to-interference improvement may be gained in the process. Dixon, "Spread-Spectrum Systems" (second edition, 1984) by John Wiley & Sons, Inc.
Spread-spectrum communication may be based upon direct-sequence, frequency-hopping, or hybrid modulation formats. The term "direct-sequence" is defined as a form of spread spectrum modulation wherein a code sequence is used to directly modulate a carrier. Id. In direct-sequence spread-spectrum communication, the modulation of the carrier occurs with a digital code sequence whose bit rate is much higher than the information signal bandwidth. Typically, the frequency of the carrier is in the bandwidth range of tens to hundreds of megahertz with the information signals occurring in the bandwidth range of tens to hundreds kilohertz.
The information being spread, in some applications, is digital. A need exists to spread time division multiple access (TDMA) or time division duplex (TDD) data.
When a direct-sequence spread-spectrum signal is broadcast, a receiver must be capable of first acquiring the signal and then tracking the signal in order to accurately despread the information being carried. This problem is compounded in a TDMA spread-spectrum system because the transmitted signal is only present in bursts for short periods of time. This represents two separate design problems--i.e., a problem of acquisition and a problem of tracking. Most spread-spectrum systems divide the incoming signal into three hardware paths. One for acquisition, one for tracking and one for data demodulation. The tracking loop is active even during data reception and is continually updating the pseudo random sequence generator (PRSG) of the data demodulation loop. Because of the non-continuous nature of TDMA or TDD signals, these conventional approaches are not suitable.
Conventionally, acquisition of direct-sequence spread-spectrum signals can occur in one of several ways. These approaches are summarized in Rappaport and Grieco, "Spread-spectrum Signal Acquisition: Methods and Technology", IEEE Communications Magazine, June 1984, Volume 22, No. 6 (pp. 6-21). One approach is the utilization of matched filters and active correlators. A second uses a serial search technique. Other approaches include variable dwell time schemes, estimation methods, and two-level schemes.
One problem plaguing spread-spectrum communication systems is the length of time it takes to acquire the transmitted signal by an individual receiver. The Rappaport and Grieco article specifically recognizes that this problem in spread-spectrum communications usually consumes a significant amount of time. A need critically exists in telephony applications to quickly acquire and track the spread digital data with small and inexpensive circuitry located in a portable telephone (PT). This problem of acquisition is caused by the lack of synchronization between the spread digital signal and the locally generated code in the portable telephone which is used to de-spread the incoming signal. For proper despreading to occur, the locally generated direct-sequence code must align or synchronize with the transmitted direct-sequence code in order to successfully despread the digital information carried. Receivers can power up at any given time and, therefore, the locally generated direct-sequence code can arbitrarily start at any time with respect to the spread signal.
A second problem pertains to the fact that the clocks of the transmitter and the receiver having normal operating tolerances are typically misaligned. For example, the clock in the receiver may be faster or slower than the clock at the transmitter. This must be accounted for during the step of acquiring, but becomes critical during tracking of the incoming signal. If the receiver and transmitter clocks are extremely accurate, the problems of acquisition and tracking become less of a concern. Extremely accurate clocks are not low power, low cost, or small in size and, therefore, not suitable for a large number of highly portable and compact receivers. A need exists to obtain acquisition and tracking in receiver environment using low cost, low power small clocks in circuits.
Hence, the twofold nature of the problem. First, the desire to quickly and with inexpensive circuitry acquire the spread TDMA/TDD digital signal and, secondly, the continued successful tracking after acquisition despite drift in the clocks between the transmitter and the receiver.
3. Results of Patentability Search
A search of issued patents resulted in the following:
1. U.S. Pat. No. 4,587,662--Langewellpott PA1 2. U.S. Pat. No. 4,984,247--Kaufmann, et al. PA1 3. U.S. Pat. No. 4,841,544--Nuytkens
The Langewellpott patent sets forth a TDMA spread-spectrum receiver with coherent detection. The Langewellpott system utilizes indirect-path signals in a receiver for fixed and mobil transmitter-receiver stations of a TDMA spread-spectrum digital radio system utilizing coherent detection. At the beginning of each time slot, a synchronization preamble is sent (once in a preferred embodiment and twice in a second preferred embodiment). The sync preamble is a specific pseudo random sequence that is the same for each slot. A correlator is used such as a matched filter. Sixteen other matched filters detect a specific pseudo random sequence that corresponds to sixteen four-bit characters. The sync-tracking correlator therefore serves to synchronize the time slots in the event of drift between the clock of the transmitter and of the receiver. Langewellpott requires a synchronization preamble every time slot and the use of two series-connected correlators. The first correlator serves as a delay line for the second correlator. The result of the first correlator is fed to an envelope detector which is followed by a peak detector and a reducing stage. The result of the second correlator is fed to an envelope despreader. The peak detector detects the absolute maximum of the correlation peaks while the second peak detector determines the times of arrival of the peaks exceeding the threshold value. Hence, synchronization and continuous tracking is obtained by this. Langewellpott requires the use of separate correlators which significantly adds to the cost of each receiver.
The Kaufmann approach provides a digital radio transmission system for a cellular network using the spread-spectrum method utilizing frequency division duplex information. The Kaufmann approach is not suited for TDMA acquisition and tracking.
The patent to Nuytkens sets forth a digital direct-sequence spread-spectrum receiver. This approach is not applicable to acquiring and tracking TDMA digital information.
4. Solution to the Problem
The present invention provides a novel system and method for quickly acquiring a spread digital signal containing TDMA or TDD digital information which is spread by direct-sequence codes. Once acquired, the present invention, through a novel and unique process, tracks the incoming spread signal in order to continually despread the TDMA or TDD digital information.
A pseudo random sequence generator (PRSG) is designed so that its random sequence output can be moved in time to match the random sequence in the received spread signal. The received spread spectrum signal is then acquired, under the teachings of the present invention, by adjusting the PRSG in such a manner as to insure that the incoming signal strength of the desired signal has been sampled at a point in time when the transmitted signal is present. The PRSG is then adjusted to the point in the sequence that corresponded with the maximum signal strength. Two types of sweeps are required to acquire the signal. The major acquisition sweep approximately locates the peak within a frame and the refinement sweep precisely locates the peak and identifies the frame and time slot boundaries.
Tracking is similar to the acquisition process. The amount that the PRSG is to be adjusted is smaller in tracking because it must adjust only for the amount of drift between the transmitter and receiver clock that has occurred since the prior frame. To accomplish tracking, header bytes are used at the beginning of each time slot. These header bytes allow the tracking to be completed at a point in time prior to the data portion being received. The number of header bytes are further minimized by having a tunable clock which is adjusted to run faster or slower depending upon how the PRSG was adjusted to find the signal peak.
The present invention accomplishes acquisition, tracking, and data demodulation with shared circuitry thereby resulting in an inexpensive receiver. Unlike Langewellpott, the same circuitry is used for acquisition, tracking and despreading of the data.